


Childhood Fire

by CrazyCatMeow



Series: Demon Tails [1]
Category: Ao no Exorcist | Blue Exorcist
Genre: Cats, Demon Baby, Demon Rin, Demon Yukio, Gen, Shapeshifting, Tails, deaged Rin, instincts, slight AU
Language: English
Status: Completed
Published: 2019-01-24
Updated: 2019-07-30
Packaged: 2019-10-15 14:47:11
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 10
Words: 13,915
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/17530721
Author URL: https://archiveofourown.org/users/CrazyCatMeow/pseuds/CrazyCatMeow
Summary: When Rin gets deaged and the sword is weakening how will Yukio handle a baby Demon.





	1. 1

**Author's Note:**

> Shiro did not die in this he is in a coma.

**Notes for the Chapter:**

> I am ignoring the manga for now in the fic. Takes place after the end of the Anime.

“Oh you poor thing, trapped inside a body so much older than you really are,” The demon says in a quiet soothing voice, picking Rin up and gently rocking him. She looked a lot like a human, but her eyes were blood red and she also had horns arching up and curled like a ram's and she had her dark blue hair styled up into a bun.

 

“Put me down! What did ya mean older? I’m 15,” Rin responds struggling to get out of her arms his tail lashing out and hitting the air.

 

“Right, a 15-year-old demon should just be starting to walk around without help, A demon your size and level would be around 150” She teases bopping Rin on the nose who let out a growl. “A little one like you has no right to growl at me I did take care of most of your older brothers,”

 

“Let him go, now!” Bon yells running over and stopping a bit away from the demons as getting any closer would mean getting hit by a tail. He had startled Rin once and had gotten hit as a result, and he did not want to repeat that experience.

 

“Oh, fine I’ll just go back home and don’t expect to hear from me again unless you need someone to help. I’m only trying to help and, Rin because your friends are all glaring at me and oh, some have guns as well I’ll just part with this. Ega Reporp Ot Trever,”She says in a loving manner that fades into deadly serious. She then gently puts Rin on his feet and scratches him behind his ears. Then she disappears in a flash of light.

 

“Well this mission was a bust,” Shima complains as they all walk back to the van that would Bring the exwires back the Academy. “Rin, you ok? What did that demon do?”

 

“I don’t know but that clown will probably want to know.” Rin answers rubbing his arms, “Is it just me or is it getting really cold.”

 

“Rin, just get in the car,” An exasperated Yukio says.

 

The exwires had been sent on a mission to find and capture a demon who was rumored to be some kind of nursemaid demon that had been stirring up trouble in a nearby town, she would show up and take children to play with them for a bit then give them back to the parents completely fine. The Vatican had seen her as a threat so Mephisto sent the Exwires to capture her alive. When the first holy water grenade went off she attacked but the instant she saw Rin she stopped attacking and picked him up. Which didn’t make sense he was only half human and she had been ignoring demons so why would she get so attached.

.........

“From your descriptions, this demon sounds like Shax, she is a high level and was, well is a caretaker for the younger of the Baal. What did she say exactly?” Mephisto says all sign of the clown gone replaced by calm and deadly seriousness.

 

“She said, Ega Reporp Ot Trever.” Just as Rin says this he begins to glow slightly. Mephisto seeing what is about to happen shoves every one except for Yukio out.

 

“Explain What is happening to my brother. Now!” Yukio shouts as Rin starts to shrink.

 

“Shax’s made it so he’s becoming the age he would look if he was a full demon she is very overprotective of the sub-kings and is probably a little bitter about not getting to take care of him. He will have all of his memories but, he may act a little differently.” Mephisto says leaning over the half-unconscious One and a half-year-old. “He will also have a bit of a problem with walking.”

 

“Why problems walking?” Yukio asks but never got an answer because Rin sat up before Mephisto had a chance to answer.

 

“What happened, Rin asked all groggy but immediately snapped out of it when he saw his hand, “T-that’s not right,” Instead of freaking out and screaming like the others in the room expected Rin simply latched himself on to Yukio’s leg and looked up at his now older brother appearance wise. On the inside, he was in full-blown panic mode wondering why and what had happened, Yukio looks torn between crying and laughing at what has happened. Mephisto was just flat out laughing at Rin’s baby lisp.

 

“We must fix those clothes be for they fall right off you,” Mephisto snaps his fingers and Rin is wearing a black cat onesie. “And now you to go back to your dorms, we shall continue this tomorrow. I will explain this to the others in the morning so you don’t have to,”

.......

“Rin you’re late!” Kuro Yells upset that Rin hadn’t played with him as he promised.

 

“Kuro, he’s asleep. Don’t ask why he’s like this I don’t know, I’m going back to Mephisto in the morning.For now, will you sleep with him and make sure he doesn’t fall off the bed?” Yukio Half explains.

 

“Alright,” Kuro agrees. Growing to the size of a truck, “Put Rin on my back,” He instructs. When he is in place he curls up into a little ball protecting his little charge.

 

“Looks like you got him, now I’m going to sleep,” Yukio says face planting on his bed. Way too tired to wonder why Kuro hadn’t just slept on the bed with Rin.

..........

“Get up sleepyhead!” Rin yells from his spot on Kuro’s back where he had latched himself.

 

“If you want me to get up then walk over here make me,” Yukio groans from his bed. Forgetting what had happened the night before.

 

“Geez, I wonder? Maybe because I can’t stand without losing my balance.” Rin says with a huff.

 

“What!”

 

“Whenever I try to take a step I fall down so Kuro is making me stay on his back. He also gave me a tongue bath. That was not fun. By the way, your first class starts in half an hour,” Rin says and Yukio starts his day by falling out of bed and running to get ready in five minutes.

 

“Thanks for getting me up, have you eaten?”

 

“No,” Rin says. “I was waiting for you,”

 

“Let's go down and get food after we get you dressed and your coming with me today,” Yukio says picking Rin up and caring him. He puts Rin in a gray shirt and black pants and they go down the stairs to the kitchen.

 

The brothers reach the kitchen and Ukobach is standing on a table with a bowl of cereal for Yukio and a weird multicolor goop for Rin.

 

“Hey Ukobach what’s this?” Rin asks tilting his head in a cat-like manner.

 

“It’s a genhhena dish for kids, Mephisto wanted you to try it considering the fact that your demon side is more pronounced now,” Ukobach says putting the bowl next to Rin who had to sit on the table because he was to short to reach the top.

 

“Rin we need to get you a high chair,” Yukio says which earned him a slight teasing growl.

.........

As Rin and Yukio enter the cram school that afternoon, Rin starts pulling back and trying to squeeze out of Yukio’s hold. “Rin, what are you doing?”

 

“I don’t want to go,”

 

“Why not? if it’s because of you being little everyone already knows, You where there when Mephisto said this you should remember this,” Rin just glared. “You’re not going to have to pay much attention, you’re here so we can make sure you don’t get in any more trouble.”

 

“Oh,” Rin says smiling relived that things Wouldn’t be that awkward. “Hey, I’m not that bad!”

 

Yukio puts Rin down on the desk. “There are still a few minutes before class starts do you want to try to walk?”

 

“I’ll try,” At this Yukio picks Rin off the desk puts him on the floor and they start practicing. Yukio holding Rin’s hands so he can balance. Rin’s tail was swaying in an almost hypnotizing manner. Often curling around Yukio’s legs. Which trips him so both twins were falling down.

 

“So it was true, Rin got turned into a kid,” Shura says staring at him and laughing when he sticks his tongue out. “You should go see Mephisto,”

 

“We were planning on doing that after class,” Rin says with a yoning showing off his fangs.

 

“Well, the clown wants you two there now so get moving.”

.........

“I am glad you decided to show up,” Mephisto says with a grin. “It’s been ages since I got to help my little brothers grow up. Informing them is so much fun!”

 

“Just get to the point,” Rin says deadpan.

 

“Alright, no interruptions and save questions for the end,” Mephisto says mock glaring at the twins,“The spell that was used to deage Rin damaged the seal and it’s started to unravel. Before you start to interrupt, I know you where about to, don’t try denying it. The spell should be off in a month or two. If it does not we may have to track down Shax. Anyway, the seal is unraveling slowly enough that no one is going to notice at least those who don’t have a template. If they do then you’ll have to explain some things.

Rin, you’re going to have fun with this. You are going to start growing horns, your fangs are going to get longer and sharper, your ears longer, you’re going to look like pretty much like you do when the sword is drawn. Moving on,

The subsection you will be in charge of is shadow and reality-altering demons. Which are both incredibly rare to find as both are masters of disguise. That will alter your powers slightly. You should be able to hover, shapeshift, possibly illusions and more things I can’t think of off the top of my head.Oh related to this your ‘weapon’ it’s not actually a weapon but can be used in a similar way. It’s nearly indestructible,” Mephisto shouts pulling out what appeared to be a giant blue-gray pillow.

 

“Mephisto, just how is that a weapon?” Yukio asks exasperatedly.

 

“Did you not hear what I just said, also save the questions for the end,” Mephisto says. Looking at Rin who seems to be having trouble staying awake. “This next bit is less obvious and has been going on for a while but must have gone unnoticed. Yukio have you noticed Rin sleeping more than normal, well for a human,” Yukoi nods confirming this. “That has something to do with The Seven Deadly Sins, most upper-level demons are aligned with one and his is the rarest Sloth! This does not have much of an impact on a normal day to day life just sleeping more and finding it more difficult to do things that don’t interest him. That will be put in his file”.

 

 

“Wate, he has a file. Sorry,” Yukio says then gets a silent threat from Mephisto.

“I’m sure you have realized that I can turn into certain animals as can Amaimon; Rin, don’t fall asleep you can sleep later. Anyway, Rin your animal is a cat. My animals is a dog specifically aterrier. Amaimon's is a hamster. So when learning shape-shifting go for a cat first it’s a lot easier. Ask Kuro for more specifics. Now questions,”

 

“First something I have been wondering about, do you know why Rin doesn’t have a problem with this?” Yukio asks petting the now sleeping child.

 

“Simple, he is how is how he is meant to be, his instincts won’t let him feel like something is wrong,” Mephisto answers.

 

“Ok, the pillow, What is up with that,”

 

“It is like my umbrella a power focuser, it will help control power and not destroy everything when he gets more power, he will almost certainly sleep on it. He will probably float on it as well,”

 

“Why is he having trouble walking?”

 

“He is a cat demon which changes the fact of hiding his tail since the seal is wearing off he will need to use it to balance. Kuro should help with that,”

 

“Thank you. I now have a lesson to teach. So goodbye,” Yukio says leaving the office with Rin sleeping on his shoulder.

 

“I’ll send some stuff to your dorms,” Mephisto calls after them as they leave. “Kuro do you know what Rin will need?” Kuro nods his head confirming that he had heard and understood the entire thing from under the desk. “Kuro you will be in charge teaching him about instincts and shapeshifting,”

 

“Sir, I can’t shapeshift anymore, I lost my fangs,” Kuro says sadly. Curling his tails around himself.

 

“I shall see what I can do about that little detail,” Mephisto says indifferently. Waving his had as if dismissing someone.

 

“Thank you, Thank you, Thank you,” Kuro shouts. "Sir why didn't you mention the instincts?"

 

“Simple it is funnier this way. Alright shoo, I have work to do,” Mephisto says fully intending to ‘work’ on watching Anime.


	2. 2

“Rin, are you ok,” Shiemi asks a bit concerned for her now pint-sized friend. Who was sitting in his normal seat the day after meeting with Mephisto.

 

“I’m ok just bored, I can’t walk so if I want to go somewhere I either have to get carried or crawl. It’s kind of frustrating,” Rin says going on a mini rant.

 

“Yeah, I think we are all adjusting to this, But you definitely have to worst end of it,” Shiemi says wincing a bit remembering what had happened the day before. When he was using the edge of one of the benches to stand and Shima stepped on the tip of his tail which caused the fur to puff up and blue sparks run through it in an attempt to be intimidating. Which completely failed due to Shiemi picking Rin. Who was whimpering like a kicked puppy. Shima was apologizing over and over again. ‘Hey man you ok, I’m so sorry,’

 

“If you two would be quiet you are still in class,” Yukio calls from the chalkboard. “Rin, even if you don’t have to pay attention the rest of us do, so please try your best to be quiet,” A few minutes after saying this a soft thump sounds as he falls asleep. Yukio gives a slight smile rolling his eyes at his brother.

............

It’s the middle of the night when Yukio is jolted awake by Kuro jumping on his face. “What is it!” He reaches around blindly for his glasses. Thinking that they’re under attack.

 

“Rin he-he’s, oh something’s happening,” Kuro says pushing Rin on to Yukio’s bed.Rin was half under the pillow that Mephisto gave him, shaking, whimpering and yelping softly, tail wrapped tightly around himself, almost like a security blanket.

 

“It looks like he had a nightmare he would always have them as a kid, this one pretty bad though” Yukio explains waking his brother. Who tackles him as a sobbing mess.

 

“It’s all my fault,”

 

“What’s your fault?”

 

“Dads gone and it’s my fault,”

 

Ah, he still feels guilty for what happened. “He’s not gone, we can go see him tomorrow if you want. he is still asleep, but we can go see him, nothing is going to stop us.”

 

“Thanks, Yukio,” And he’s is out again.

 

“Kuro you go back to sleep I got Rin tonight,” the twins curl up together as they did as kids. Rin’s tail acting as an anchor wrapped around Yukio’s arm. Kuro joins the two sleeping on Rin’s chest.

............

“Rin, get up!” Yukio yells. He had been trying to get him up for a solid ten minutes now. Eventually, he gives up and starts to get him ready while he’s asleep. It’s going well until Yukio tries to tuck Rin’s tail around his waist. The thing just unravels its self Yukio gives up again and just brings Rin down to the kitchens, where Ukobach was waiting with food.

 

“Is he not waking up?” Kuro asks cocking his head.

 

“Yah, now that he’s so small he’s not difficult to move,” Yukio answers starting to eat. He had put Rin on the floor next to the table.

 

“He’s a sloth, cat demon, right? It’s normal for them to sleep for a long time then wake up when it’s important,” Ukobach explains. Once Yukio leaves with Rin he goes to Mephisto and tells him what's happening, that the seal on the sword is wearing off faster than they previously thought.

............

“I have good news for you two.” One of the nurses at the hospital says to Yukio. “At around three in the morning your dad woke up, I’m sorry we didn’t call we didn’t know your phone number.”

 

“Really that’s great!” Yukio says trying not to yell. “That’s all right we were having some problems, and visitors aren’t allowed at that time right?”

 

“What’s going on?” Rin asks a bit confused and disoriented, having been sleeping for at least fifteen hours all the way up until the door of Shiro’s room.

 

“Dad, he’s awake,” Yukio says hugging Rin.

“Really! That’s great.” Rin says excitedly.

 

“Really. I need to go grab something, Rin go ahead and see him,” Yukio instructs Rin who gives him one off those looks that says if I could I would already have. “Sorry,” Yukio sets Rin down on the bed.

 

“Dad are you up?” Rin asks scooting closer to Shiro’s face.

 

“Yep, I’m definitely dead, one of my fifteen-year-olds is one?” Shiro groans lying back on the pillows. “How does that even happen?”

 

“According to Mephisto this is how old I am as a demon,” Rin tries to explain.

 

“So you know everything? I’m so sorry for not telling you sooner. Fill me in on what happened while I was out.” Shiro says hugging Rin. He tells him everything from the start all the way to the Impure King. Leaving out some important details and names. “Well you are very accomplished, now where is your brother?”

 

“I don’t know, He said that he had to go get some things,”

“That I did but not something someone,” Yukio says from the door letting Kuro out of his bag. “Rin promised to bring him to see you eventually and since you are awake what better time,” Kuro was now on Shiro’s chest purring like a lawnmower. Then Kuro started purring?

 

“Rin? Are you purring.” Shiro says stifling a laugh.

 

“Um, I don’t know,” Rin says voice vibrating. “How do I stop,”

 

“Yep, definitely purring.” Shiro teases, “Yukio do you know how this happened? Rin didn’t explain how he got like this, all he said was it’s how old he is as a demon.”

 

Yukio looks up and starts to explain. “We were on a mission to stop a powerful demon known as Shax, she is a caretaker for the Baal. she used a spell to revert Rin to his demon age. Mephisto was kind of helpful in his own way. He vaguely explained what happened. The seal on the sword is wearing off so he should get more and more demon like. Apparently, Rin can float, he is a cat based demon, and his Seven Deadly Sin is Sloth,”

 

“That was not what I expected. Now, Rin Okumura whatever are we going to do with you,” Shiro says tickling the living daylights out of Rin. Who started yelling for dad to stop in between laughing fits. “Are there any problems I should have known about.

“No!” Rin yells the Instant Shiro had stopped talking.

“Yes, Rin can’t walk, he can’t balance with his tail,” Yukio answers which earned him a few feral sounding growls.

 

“Well, that’s not good. Have you been trying, Rin?” Shiro asks.

 

“Yes, I have I just can’t get it,”

 

“Have you tried to stand and stay up,” Shiro asks deep in thought.

 

“Ya, I can stand I just can’t walk,” Rin answers curling up against Shiro. “It’s hard to get up I normally have to hold on to something to get up.”

 

Shiro suddenly sat straight up Kuro who had moved to his lap let out a hiss of fear and anger, “Rin, Yukio, you might want to go home now, he’s coming,”

 

“Who is he!” Rin yells tail standing straight up.

 

“Mephisto’s little brother,” Shiro answers deadly serious.

 

“Amaimon?” Yukio asks in confirmation raising an eyebrow.

 

“You know him,” Shiro asks also raising an eyebrow.

“The demon I fought in Mepphyland and the forest, yeah that was him,” Rin says completing the family with another raised eyebrow.

 

“Rin! You need to tell me these things,” Shiro says A bit exasperated this is some left out this important detail.

 

“Sorry,” Rin says looking down at his lap a bit ashamed.

 

“I head from Mephisto that my littlest brother was even littler now,” Amaimon says hanging from the ceiling eating a lollipop. “I didn’t really believe him till now, he also sent me to ask when you’re going to get discharged, Paladin of the True Cross.” By this point, Amaimon had grabbed Rin and was petting his head and Rin starts purring again. Hugging his older brother and curling his tail around his wrist, giggling.

 

“I get discharged in a week,” Shiro answers rubbing the back of his head. “I did stab my self in the chest, so even after that, I’m on constant watch. For at least a month, they probably won’t let me do much besides paperwork.”

 

“Great,” Amaimon says setting Rin down on the floor gently how Mephisto had told him to handle little demons and jumps out the window.

 

“Rin? You ok.” Shiro asks the kid who is desperately trying to stand up without help, a feat that normally requires a little or a lot of help.

 

“Yah I got it. Just give me a minute,” Rin says swaying on his feet a little. Kuro jumps off the bed comes up next to and he stops in front of him and tells him to stop focusing on keeping his tail still and to let it move wherever it wants to.

 

“Kuro? Why should he let his tail move,” Yukio asks confused that Kuro was saying such a thing, Rin needs to keep his tail hidden. 

 

Kuro gives him a pointed stare. “Cats need there tails to balance, without one your balance is gone.”

...........

“Kuro you should never have told him that,” Yukio says sitting down in the dining area at the dormitory exhausted after chasing Rin who using what Kuro had said run around for the rest of the day.

 

“I’m sorry,” Kuro says. “But he needed to know how to walk,”

 

“Speaking of Rin, can I have him for the night?” Mephisto asks poofing into the dorm room.

 

“That’s up to Rin, but it’s good with me,” Yukio answers not looking up from his book. “He should be asleep upstairs,”

 

“Thanks, little brother, I’ll bring him back some time tomorrow,” Mephisto says running up the stairs like the maniac he is.

 

“Rin, get up!” He yells starling the young demon.

 

“I’m up! Oh, it’s you. What do you want?”

 

“I’m offended, well tonight we are going over to my place to discuss some things. Eins, Zwei, Drei,”

.........

“So why did you bring me here?” Rin asks a bit annoyed at having been woken up.

 

“Am I not allowed to see my little brother?” Mephisto asks coolly.

 

“Really. You actually caring about me doesn’t seem all that believable.” Rin says unbelieving that Mephisto would do that without a proper motivator.

 

“Ok fine Rin, I brought you here because your horns are about to start growing in. When they finish growing in they will be magnificent but for the time being you’re about to be in unimaginable pain.” Mephisto says leaning on his desk head resting on his hands.

 

“How much pain?“ Rin asks shaking slightly.

 

After a few minutes of thinking of how to describe what will be happening to Rin, Mephisto answers. “You remember when Amaimon cracked your swords slightly. Think around that level of pain when they start to push through the top of your head. Actually much like when you humans, well not you anymore, start getting your new teeth. It hurts a lot when they start coming in but after the first stage the pain lessons quite a bit. More like you walked into a wall. When the horns start to grow in your fangs and ears should get longer and sharper. Be careful not to cut your lips.”

 

“Do you know what they will look like?” Rin asks his ears twitching in irritation.

 

“Not exactly but they should look a little like rams. Little kitty.” Mephisto teases bopping Rin’s nose.

 

“What does that even mean,” Rin growls out snapping at Mephisto’s fingers.

 

“Exactly, now time for bed,” Mephisto Snaps his fingers and Rin is wearing blue pajamas and a grumpy expression, but he falls asleep in a matter of seconds. Once he’s asleep he is put in a crib in Mephisto’s room and sleeps until morning. Mephisto, however, was going over things that Rin will need. ‘He should need something to chew on so his fangs don’t hurt him as much when they grow in. If his claws get any sharper he will need to scratch things so get a scratching tree? When he figures out how to shapeshift for the first time he will probably need cat toys, well he will want those ether way. When his instincts start showing up he’s going to hang off of people and be overly defensive, he also will probably love nature so have Shiemi take care of him in the garden sometimes. Have playtime with Kuro and if Izumo agrees to have her and Shiemi’s familiars come as well.’ “Sleep well little one. when you wake you will be in for a world of pain."

**Notes for the Chapter:**

> *(you know the baby’s who are almost narcoleptic.)  
> **(Yukio has just enough demon blood that he can understand high-level demons he was able to do this as a kid as well.)  
> ***(Rin and Amaimon have are friends,)  
> Ok, so I'm thinking of updating every Thursday, or if the chapters not finished ever other is that good?


	3. 3

As Rin woke up is head was throbbing slightly and something told him not to move much or it would hurt even more. “Meffy! Are you up?”

 

“Rin, it’s noon, you’ve been sleeping for a long time, how do you feel?” Mephisto asks picking Rin up.

 

“My head hurts a little, what’s that?” Rin answers distracted.

 

“That is a common toy for little Demons,” The toy in question looks to be a lime green spike ball and even though the spikes aren’t particularly sharp they will hurt if thrown at someone. “Do you want it?”

 

As an answer, Rin carefully nods his head and reaches for the ball. Mephisto hands it to Rin who starts squeezing it and when it squeaks he grins. “Thanks, Meffy...What’s going on? Why am I acting as I did as a kid?”

 

“Well, it’s your instincts you’re bound to act your age occasionally.” Mephisto answers. “If your lisp gets any worse let me know. Also, open your mouth for me,”

 

Rin complies opening his mouth. Mephisto peers into it inspecting his fangs. Then he moves his hand to Rin’s head and rubs around looking for horn nubs. “Ow, stop that hurts!” Rin yells when he finds them and runs his fingers over them.

 

Mephisto jerks his hand away “I'm sorry, I didn’t think they were this sensitive.”

 

“It’s ok,” Rin says rubbing his head to try and lessen the pain.

 

“If it makes you feel better the sensitive will fade,” Mephisto says shrugging.

 

“Ok, can I go home now?” Rin says starting to chew on the ball.

 

“Not quite yet I need to ask you something,” Mephisto says setting Rin down on his desk. “Have you noticed that you’re clinging to people more?”

 

Rin thinks for a second then answers, “A little, when we were leaving the hospital Yukio had to peel me off of Dad,”

 

“Hahahahaha. That’s completely normal, young demons do tend to cling to older siblings and parents. How old were you when you started to walk?”

 

“According to Yukio and Dad, I was around three. I had balance problems,” Rin says eyes getting wide when Mephisto grins.

 

“That is helpful, and after the Koma Sword was released,” Mephisto questions leaning forwards.

 

“I was pretty out of it, but after I got the tail I didn’t feel like I would fall over after standing,” Rin answers. “But I did have to relearn walking

 

“I’m going to take you home now, I’ve replaced some of the things that needed to be changed due to your current age now shoo,” Saying this Mephisto picks up Rin and puts him outside of one of the main school buildings.

..........

“Did you do the homework Mr. Okumura gave us?” Shima asks as he walks with Bon and Konekomaru.

 

“Guys, do you hear that?” Konekomaru suddenly says stopping in his tracks.

 

“Sounds like a kid,” Bon says also stopping, “Who would have a kid on campus? Rin!” The trio takes off running trying to find where the sobbing was coming from.

 

 

“I got him,” Konekomaru says into his phone having found Rin and was sitting on the school stairs with the still sobbing child. “Rin, you’re okay I have you,” At a loss for what to do he gets a cat toy from his bag, it’s a stick with string and feathers. When Rin sees the toy he growls at it and starts to batt at it playfully.

 

“Rin why are you out here?” Shima asks concerned for his friend. Only to get hissed at. “You can’t be out here alone,”

 

“He hasn’t said a word since I found him, I think he got scared and is running on instincts right now,” Konekomaru explains his theory. “He’s been playing with the cat toys I have.”

 

“I’m going to call his brother,” Bon says rolling his eyes at Konekomaru.

.........

“Rin, can you come over here?” Yukio asks his brother. he had rushed him back to the abandoned dorms and ever since they had been trying to get him to say anything. He wasn’t acting on instincts now, but he’s still not talking. “What’s wrong?”

 

“Hurts,” Rin says curling himself up in a ball on the verge of tears.

 

“What hurts?” Yukio asks his worry for his brother growing

 

“Head, mouth,” Rin answers hiding under the blanket hands over his ears.

 

“Let me see,” Yukio says carefully opening Rin’s mouth. And looks at his teeth. Which were growing into fangs longer and sharper than they previously were. “Your fangs are growing in,”

 

“I know,” Rin whimpers as he starts biting the toy as he falls asleep.

 

Yukio starts running his fingers through Rin’s hair. “What are we going to do with you. Kuro, do you want to have Rin tonight I need to look up some things,” Kuro agrees And takes Rin up to the bedroom. Yukio then goes to one of his books it is one of the first books on exorcism that he got. Unlike most books of the same subject, it was about how young demons grow and develop. He only had to use it once before when Rin had eaten something slightly poisonous to demons.

 

The chapter that Yukio opens to is on ‘What To Do When The Horns are Growing In’. As he reads his worry for Rin increases to the point that he runs upstairs to give him pain killers. The book stated that mostly human demons in appearance will be in great pain when horns start to come in. So severe that most pass out from the pain.

 

**Notes for the Chapter:**

> Happy Valentines


	4. Chapter 4

It was a few days later when the nubs pushed through and instead of looking how Mephisto predicted the Horns looked almost exactly like Kuro’s. Who was excited that Rin looks kind of like him. At some point, Mephisto had stopped by and dropped off a bunch of toys, a highchair, crib a few other things and the biggest cat tree that Yukio had ever seen. Mephisto explained that having giant cat trees in a house was normal for kittens. (Baby cat demons)

 

Rin still had to go with Yukio to cram school. Rin had started to snuggle up to whoever he had to sit next to the first time he did it he was sitting next to Konekomaru. Who froze and looked down at Rin then he shrugged and went back to work. That night Yukio was sleeping when Rin got on the bed; Yukio was positive that Kuro had helped. Rin got right next to Yukio and pats his head and collapsed on his back.

The next morning the brother get ready, Mephisto had given Yukio a mini school uniform to him for Rin, who hates it. When they go downstairs for breakfast the food is set out and Rin is set in the highchair. “Rin, how do you feel?” Yukio inquires planning to do a mental analysis on Rin.

 

“I’m, ok my head isn’t hurting anymore.”

 

“That’s good. Oh, today you’re going to follow Shima, and cram school is going to be here,” As Yukio explains what's going to happen Rin’s attention was on a stray Coal Tar that had gotten in somehow. When it floated closer to Rin his attention was fixed on it. Yukio saw it a took out his gun to shoot it.

 

“No don’t hurt him!” Rin yells the Coal Tar floats behind Rin who was in the process of escaping the highchair. When he escapes he stands on the table arms out and a stubborn look on his face.

 

“Rin. it. is. a. demon,” Yukio response emphasizing each word lowering the gun.

 

“I’m a demon,” Rin stammers eyes going wide and filling with tears.

 

“Rin, I didn’t mean it like that.”

 

“I know, I-I can’t help it,” Rin manages through tears his tail drooping.

 

Yukio puts the gun down on the table and picks Rin up. “Hey, Rin stop crying no one is going to hurt you,”

 

“Mr. Okumura, What do you us to do today?” Konekomaru asks as the exwires enter to room.

 

“For now, just self-study,” Yukio says. As he brings Rin upstairs and sets him on his bed with Kuro. “Rin, I will be back in a bit just play with Kuro and don’t leave this room,”

.........

Kuro jumps off the bed and drags a toy mouse out from home underneath one of the desks. He bats it in Rin’s general direction and they both start playing with it chasing it around the room biting it. One important detail was forgotten, this particular cat toy was infused with catnip.

 

So when Yukio came back a few hours later Kuro was bouncing off the walls and Rin on his bed purring and hitting the air. “What are you doing Rin?” He asked to no reply. “Ok, I am going to give you a bath and then you are going to sleep in the crib thing Mephisto got you.” The crib its self was a dark red with a few symbols on it, Mephisto explained that some of the symbols would lessen the effects of curses and one of them was the Royal seal of the Baal.

 

After the bath Rin was pretty much back to normal. Instead of putting Rin in a different outfit Yukio put him in pajamas as Rin had fallen asleep and was purring. When Yukio puts Rin down in the crib he lets out a pitiful wine Yukio puts a few toys in as well and he quiets down immediately.

**Notes for the Chapter:**

> I’m sorry it’s so short. Had like 5 essays in the past weeks.  
> * he’s trying to figure out what age his mentality is.


	5. Chapter 5

Life was pretty much insanity, Rin had to go to a doctor because he had started complaining about his head hurting and he was behaving weirdly. The doctor that he went to was a specialist for half-demons. The doctor had said that Rin’s mental state was fluctuating between that of a demon and one of a human. Now, this caused a few problems one of these problems was languages, he would occasionally slip in different dead or demonic languages.

 

He wouldn’t go anywhere without, Yukio, or Shiro. Who was out of the hospital and back at the monastery. He called the twins every day or he called Mephisto to check up on the boys as he was about to go back to being the paladin.

 

To most of the classes amusement when startled Rin would let out a hiss or mew. Mephisto said that this means he is getting ready to shapeshift in a few weeks and would continue to happen.

 

Mephisto had also found out more about the curse on Rin. It was actually a good thing that this happened as being both human and the demon had messed up his biology. Now that he is his ‘proper’ age he can adjust to demon powers properly instead of caching on fire at random times. Well, he can’t catch on fire at all since he was a little young for fire powers. Mephisto had also sent a demon to teach Rin about gehenna traditions and geography, also about demons in general.

 

“Hey, Yukio get up,” Rin whispers in his ear.

 

“Gah! Rin don’t do that,” Yukio says sitting up putting his glasses on, “Rin what happened to your ears?”

 

“Don’t know, just woke up like this,” Rin’s ears had pulled back into the elf-like shape that they are with the sword unsheathed he was also moving his ears twitching them, unconsciously by the looks of it. “Teeth are the same.”

 

“Ok, so why did you wake me at... Rin! It’s 4 am!”

 

“Sorry, it’s just everything feels tingly and a little fuzzy?” Rin says scrunching up his face.

 

Yukio rolls his eyes “I’ll call Mephisto at a reasonable time and he can deal with that.”

 

“No need to call me little brother,” Mephisto says as he poofs in. “I’m already here.”

 

“Meffy!” Rin yells and throws himself at him.

 

“Hello, little one. So as for what is happening Rin is finally coming into his personal demonic powers,” Mephisto puts Rin down and he starts moving to his bed.

 

“Really, so what will happen to him?”Yukio asks.

 

“Well knowing how Rin handled the flames I would get a Child leash for him as floating is one of his powers that will show up first. This one is not a power and more of a nuisance, an aversion to sunlight!”

 

“So, Meffy just how do I stop floating?” Rin asks from up near the ceiling clutching his pillow as he suddenly flipped upside down.

 

“Oh my, it looks like you figured out how to go up. What were you trying to get?” Mephisto asks stiflingly laughter.

 

“I was trying to get my blanket off the bed,” Rin says as Yukio grabs his leg and pulls down him into his arms.

 

“All right I’m going now,” Mephisto says standing and moving towards the door.

 

“Mephisto, aversion to sunlight how is that going to affect him,” Yukio calls after him confused.

 

At this Mephisto smirks and says “Oh you know, he just won’t like it. It’s probably been affecting him for a while. Demons are creatures of the night so it’s only natural for us to like the dark.” Then disappears in a puff of pink strawberry scented smoke.

 

“Well, that was interesting.”

 

.................

It was around five days later that Rin had gained enough control over his gravity problems to not be monitored constantly. To a good portion of the cram school teachers, annoyance Rin had started to try to run off and find things to do. This problem was solved by Mephisto who while Yukio was teaching or in school, himself took Rin and had him start his gehenna lessons and for once Rin wasn’t having learning problems.It was during one of these lessons that the first shapeshifting incident happened.

 

Rin stands up to turn in a paper that was about classifying different demon species. Instead of normal assignments, Rin had mostly hands-on lessons. He stands only to double over clutching his stomach as Mephisto rushes over and plucks him up to set him down on the desk. In a blue puff of smoke Rin was gone and in his place, there was an almost pitch black kitten so small it can fit in the palm of your hand. Unfortunately for Rin, Kuro was at the dorms and not in Mephisto’s office.

 

“Rin are you ok,” Mephisto asks whispering as not to hurt Rin’s ears. In response, he rolls over onto one of Mephisto’s hands and gives a small purr. “I’ll take that as yes. I’m not positive but you should be able to transform back in a few hours, so in the meantime, I’ll give you to Yukio.”

 

 

...........

“My dear little brother, today I interrupt your class to bring you, Rin,” Mephisto flaunts into Yukio’s class which was his actual school class, not cram school. Pushes Rin along with a note explaining what happened into his hands and leaves.

 

The teacher looked completely aghast. “Mr. Okumura just what did directer Faust give you? Also little brother?”

 

“He gave me a kitten and He is a good friend of my family and he, sort of adopted me and my brother as his younger brothers,” Yukio says slipping the sleeping Rin into his shirt pocket and sits back down.

 

“Well, after class drops it off at your dorm, don’t bring it to another class.”

 

 

“Rin, hey wake up,” Yukio says gently shaking him with two fingers to get him up and to eat something. After a few minutes, Rin wakes up licks Yukio’s face which was around an inch away and after a few minutes of trying to walk gets is and enters the dining hall where Yukio stops him from entering the kitchen. Picks him up and puts him on the table. Then he gives him a bit of milk and a plate of little meat chunks. After eating he mews and falls asleep on the table all curled up.

 

Kuro wanders in tracking the sent of Rin and Yukio who didn’t say anything to him about coming home early. “Rin!” Getting a nod from the younger twin he scruffs Rin and brings him to the nest that he had constructed in an unused room and they fall asleep.

..............

 

When the two wake up Rin is back to being a one and a half years old and for some reason, his ears had remained like cats. After a few minutes of looking, he finds Yukio. Well, Yukio found him. At one point he had lost control of his gravity and had ended up stuck on the ceiling. Which is how Yukio found him. “So do you have an explanation for what these are doing on my head,” He says standing on the ceiling in a power stance.

 

“Demons have several forms since you’re so little Mephisto thinks that your current form represents the powers you have, to get rid of it just concentrate on getting back your ears,” with a pop Rin’s ears are back to the elven that everyone had been seeing for the last few days. Yukio grabs the small hands that are reaching for him and pulls Rin down and on to the floor in front of him.

 

“Thanks,” Rin says rubbing his eyes

 

“Hey, are you ok? You have been through a lot over the last month,” Yukio says gently brushing Rin’s hair out of his face.

 

“I d-don’t know, part of me feels right and the other half is uncomfortable like everything feels wrong and I just can’t tell what’s going on. Everything is too loud and bright, I can’t do anything to make it stop. Everything is just too much. I’m not allowed to be alone or on the roof. I can’t eat the food I want cause it’s too spicy and the weird instincts I want to scratch things and bite things. And everything is too cold!” At this, he breaks into tears that he had been holding back and bottling up for almost a month.

 

“It’s going to be ok, Mephisto is almost done with finding a way for you to go back to normal,” Yukio reassures giving Rin a hug. “You have been so brave and hopefully this will end soon.”

 

 

 


	6. Chapter 6

“Well, you’re just going to have to wait for the spell it were off,” Mephisto says spinning slowly around in his chair to face Yukio.

 

“Don’t worry littlest brother, Shax, said that it should only last a few more months at most. Then your brother going to grow up over the course of two weeks,” Amaimon trying to help explain what they had found out to the twins. “Oh, some of our Oder brothers want to meet you and Rin so don’t be surprised if they show up at any time. Don’t worry though Mephisto made them promise not to hurt anyone.”

 

As Yukio bows and starts towards the door, Mephisto calls after him. “Oh my, I nearly forgot to tell you an exorcist is stopping by tomorrow afternoon to teach a more in-depth lesson about young fire demons, and I said you would be there,”

 

“Why?”

 

“Because, although Rin will be over shadows and reality. Even though he technically doesn’t have them he still has the flames they just haven’t manifested yet, He can only spark which is impressive. He was also complaining about how cold he was. So bring him with you to the presentation.,” Mephisto says sitting at his desk and looking at some papers smirking.

.............

“Rin are you ok?” Yukio calls as he steps into the dorm.

 

“I’m doing better now, did they find a way to reverse this?” He asks hopefully sitting on the floor playing with Kuro.

 

“Rin, I’m sorry but... you’re going to have to wait a few months. On a different note, Mephisto says that Shax had no idea of how this would affect your mind and since you don’t have your fire that you would be so cold.” As Rin’s eyes fill with tears Yukio is about to do something that he and Shiro had been planning for a few weeks. “So, why don’t we go visit dad I’m sure he would love for us to go see him,”

 

Rin looks up and asks. “How will we get there?”

 

“I have a key,” Yukio says rolling his eyes at how dense Rin can be.

 

“Ok, let’s go,” Rin says with a full fanged smile. Ignoring the fact that he had just asked an obvious question.

......................

“Dad? You here.” Rin calls out into the monastery.

 

“Rin! I’ve been told that you aren’t fowling the laws of gravity,” Shiro teases as he enters the chapel.

 

“Hey, it’s not my fault,” Rin protests lightly as he starts defying gravity. “It just showed up last week, but I think I have pretty good control now.”

 

“I know kid, I know,” Shiro says messing up Rin’s hair as he pulls him over.

 

“Are Rin and Yukio here?” One of the priests calls as he leaves the kitchen entering the chapel excited to see the twins.

 

“They are And Rin is as cute as ever,” Shiro says pinching his cheeks and picks him up showing him off. Most exorcists that knew of the son of Satan had been informed of him being deaged and the priests at the monastery were worried that the Vatican would take Rin and have him killed. Instead, the Vatican had decided that the spawn of Satan would be allowed to live at least until he was back to normal when they would decide what to do with him.

 

“Put me down!” Rin yells kicking his legs. His tail was out and the tip had gone and poofed up to nearly twice the normal size with the blue sparks that would show up every so often when he got worked up.

 

“Okay, okay don’t blow up,” Shiro says setting Rin on the floor as they all go into the more home-like areas of the monastery.

 

As Rin wanders around the sitting room his tail brushes past a cross that was down on a table for cleaning. As he lets out a hiss of pain in reaction to the object and starts to sniff back tears, the others look over and start to move all such objects out of his reach. His tail had suffered a slight burn and Yukio was trying to rap it in a was that he wouldn’t get more hurt or impair his movement. Shiro was looking over books that had information on half demons and reactions to religious objects.

 

“Hey, Rin How does that feel?” Yukio asks when Rin finally stops crying.

 

“Ok, but it still hurts,” He answers rubbing his tail.

 

“That’s to be expected, Rin it’s not going to stop hurting instantly,” Yukio says trying not to laugh at his brother.

 

Shiro comes over and puts Rin on his lap. “I’m sorry, but I’m going to have you stop going to the classes in cram school that focus on symbols and objects as well as the aria class. It’s going to be up to Yukio if you can enter his class on any given day”

 

“Why not, won’t that stop me from becoming an exorcist?”

 

“Because according to these books as a half-demon this puts you at risk to actually be exorcized by them. I don’t think you want to go to Gehenna.” At this Rin’s eyes go wide and he tries to get as close to Shiro as possible.

 

“Why is this happening I’ve been fine with this stuff before?” Rin groans burying his face in his hands.

 

Yukio stands and takes Rin’s hands and starts to explain. “It’s because of the swords seal. For most of your life, it’s been active and nothing happened but since it’s weak more and more things will start to affect you.”

 

“This gets better and better,” Rin says wiggling out of Shiro’s arms and floats up to the corner of the ceiling.

 

“Rin What are you doing?” Shiro asks exasperatedly.

 

“Dad, it’s how he’s started getting out of difficult or frustrating situations,” Yukio says groning at his brothers' antics that had gotten old real fast.

 

The rest of the visit was spent with few problems. Only once did something that can only be considered unfortunate happens. It was time for lunch and Rin decided that he wanted to help cook not realizing that his hands were too small and pudgy to be of any help. He had almost cut his finger and was unceremoniously take it off the chair he was standing on and placed outside of the kitchen.

Where Shiro picked him up a few minutes later and brings him into his office to talk.“Rin I want you to be honest with me, how has Mephisto been treating you?”

 

“Well, he’s been Mephisto?” Rin says shrugging not having a better way of describing it.

 

“Fair enough,” Shiro says shrugging.

 

“So, why are you asking?”

 

“Well, as you know he is Samael and is your older brother. Has he been treating you differently since this happened?”

 

“He’s been more tolerant about me not getting things right away and he lets me hang out around him.”

 

“That’s good. So, I am going to come to the cram school tomorrow and teach a lesson on fire demons. Yukio already knows and you’re coming with me.”

 

“Really? I get to go and watch a lesson!” Rin yells excitedly to Shiro’s surprise after a moment realization dawned on his face.

 

“Right, you can’t go to lessons right now. Yah, you're coming with me actually on Mephisto’s request. He said it might do you some good to interact with other demons that also use fire.”

 

“Cool,” Rin says excited that he gets to meet another fire demon outside of combat.

 


	7. Chapter 7

“As most of you know I am the paladin, as the leader of the exorcists your director has asked me to come here and teach you more about fire type demons,” Shiro states as he speaks to the exwires and Yukio. “If a demon can use fire it is classified as a fire demon by us exorcists. This is because of how the flames work when a demon uses its flames it puts some of its power into the flames. The hotter and sunnier the day normally means more powerful flames. Now, this is not always the case if there is a shadow demon who can use flames they use their own power to fuel their fire as most don’t care to venture out during the day For an example Rin here. If you’ve noticed he doesn’t go outside much since he was deaged.” Shiro pauses here to scoop Rin Into his arms.  
“Now, The demon king of fire Iblis-” Shiro stops talking again when a pillar of fire erupted in the middle of the room. His face drops into a deadpan.

“You called,” A demon says shrouded in flames. He was wearing all black and red robes with a crown of fire on his head. He had flaming red hair and black eyes a stark contrast to his pale skin

The click of a gun being locked and loaded echoed through the now silent room. “Who are you!” Yukio demands. 

“Why, my own brother doesn’t recognize me,” the demon says with a hurt expression.

“Iblis?” Rin asks turning his head to the side. “Is that who you are?”

Hearing this the now named Iblis turns around to face Rin who was being held by Shiro who had picked him up when the fire appeared. “Rin? Mephisto told me what had happened but I didn’t think he was telling the truth. Before any of you attack, I promised that I wouldn’t harm anyone here.”

“How do we know we won’t hurt anyone?” Yukio says not lowering his gun. 

“The seals on campus will allow me in under the promise of peace. Take notes on this kids, if a high-Level demon promises something they are bound to keep it,” Iblis says as Rin worms his way out of Shiro's arms and walks up to him and puts his hands up in the air in a pick me up gesture. Iblis picks him up and hugs him. Twirling him around he says, “So any questions on fire demons if so I will be happy to answer them.”  
......

Most of the questions asked had been answered and only two people had their hands up. “Yukio, what’s your question?” Iblis selects holding Rin who was falling asleep.

“My question involves Rin as you may know more on this than Mephisto does as you are the demon king of fire. Rin is constantly saying that he’s cold, Mephisto said that it has to do with him not having access to his flames because he’s so small. Is that actually the problem?” Yukio asks distrustfully. 

“Well, kind of since he didn’t have flames until the seal was broken his body had to rapidly adjust to the flames so he didn’t burn himself you know. Since he doesn’t have access to his flames his body is trying to compensate but can’t get to the correct temperatures. Which is probably why he decided to have me hold him. I’ve been told that I’m hot.” Iblis answers snapping his fingers into a gun on one hand on the last sentence. As he was still holding Rin. Then he runs his hand through his red hair. At some point, his clothes had changed from the extravagant robes to a suit and tie. “Moving on, the last question goes to Shiro, what can I do for you paladin?” This last bit is said with an extravagant bow.

“In demon terms how is Rin doing developmentally wise?” Shiro asks concern for his adopted son bleeding through his tone.

“He’s actually a bit advanced, but he is a half demon so that may be why, but it’s hard to tell he will probably stop aging at twenty like the rest of the kings,” Iblis says sitting down in a chair that he had pulled out. 

As the room cleared save for Yukio Iblis scoops him up when he got close enough so he has both of the twins in his arms and takes in their sent. 

“What are you doing?” Yukio asks completely weirded out. 

“Right, you weren’t raised in Gehanna. Well like it or not we are family and demon families rely heavily on sent,” Iblis explains setting Yukio down. “So I’m making sure I have yours.”

“But, how demons are the literal definition of evil,”

“You’re thinking of demons from more church-based things, Gehenna is not hell and although some names are the same we are not the same. Why your Holy objects and phrases affect us like they do we’re not entirely sure. Something about Believe being strong enough to make things real, we don’t know. And Like humans some demons are good and some aren’t. Father, is neither he is fairly Morley gray. He cannot come to Assiah without going insane people here fear him so much that it drives him to insanity nearly instantly. I was sent in-part to meet you and get to know you and partially to formally apologize for all that has happened. Here is the letter that was sent,” Iblis says handing Yukio a letter that was sealed with blue wax. The letter was read then unceremoniously shoved in his pocket.

It was at this time that Rin decides to kick and whine to be set free from his older brothers grasp. Iblis relaxes his grasp on Rin who when released stands up and jumps down to the floor and starts polling On Yukio’s sleeve. “Can we go get food now I’m starving?” 

“How about we head back to the dorms and continue this there?” Iblis suggests standing up and picks up Rin by his foot which made him giggle and he moves to put Yukio on his back in a piggyback. 

“Don’t touch me,” Yukio snarls backing away towards the door.

“Yukio? What’s going on why are you leaving?” Rin asks as he and Iblis watch.

“Rin! Do you really think a demon would tell the truth. He probably killed the guards and broke the seal,” Yukio cries. 

“Demons Lie to humans not to other demons,” Iblis explains slowly walking up Yukio who had ended up huddled in the corner. 

“What’s happening?” Yukio screams as his teeth sharpen a little. “I’m not a demon,”

“Yes you are, It appears that your demon blood is activating. Rin, will you be able to hang on to my jacket we need to get back to the dorms before he gets any more freaked out. Yukio I need you to answer me, where does it hurt?” Ibis says scooping the boy up into his arms.

“It feels like I broke my tailbone,” He whimpers out. “And my teeth hurt,”

The flames that Iblis entered with reappearing and the three of them vanish and reappear in the dorm. Yukio is set down on his bed on his stomach. At this moment Mephisto and Amaimon enter, Mephisto to help with Yukio and Amaimon to watch Rin and give him food and then return.  After the two of them leave Mephisto and Iblis sit down and contemplate what’s happening.

“What’s happened?” Mephisto asks a bit of concern in his voice.

“His demon side is becoming more prominent and I believe it is causing him to grow a tail and fangs,” Iblis says.

“Well that’s not good, regrowing a tail is painful and this kid is starting from nothing,” Mephisto says running his had through Yukio’s hair.

“Can’t you speed up time on his body? You did that for me,” Iblis says uncoiling his own tail which has the same shape as Rin’s, but was covered in blood red scales. 

“Normally I would, with the proper price, but since he isn’t I can’t fast forward time on him he needs to be able to acclimate to it. He isn’t cat based so walking shouldn't be as hard as it was for Rin,” Mephisto explains.

“Do you know what demon type he is?” 

“I think his seven deadly sin is Pride. you should be happy about that, you have something in common with the kid. As for his animal, I believe him to be some sort of owl,”

After a few minutes of sitting in silence, the older brothers look at the door Rin and Amaimon should be back by now. “I’ll go check on them,” Iblis groans standing up and walking towards the kitchen. When he enters Rin and Amaimon where face to face in a vicious play fight as a cat and hamster. “Oh, I want in,” Iblis says turning into a small red dragon only a little bigger than Rin. When Amaimon notices He And Rin gang up against Iblis. The brothers mess around for a bit and when they finish Rin and Amaimon have successfully pinned Iblis to the table. Iblis and Amaimon turn back with no problem but Rin has yet to master turning back he can shift into a cat perfectly but he has trouble turning back. After a few minutes, he decides to move onto Iblis’ shoulder and they all head back upstairs where Yukio and Mephisto where. 

“So, what’s going on? Is Yukio ok?” Rin asks frantically as they enter the room to see Yukio lying on his bed drenched in sweat.

“Rin, how your tail appeared; he has to grow one,” Amaimon says gently petting his head.

“And thankfully it’s growing in correctly if it wasn’t then we would probably have to call Lucifer here to have one of his doctors fix it,” Mephisto chimes in.

Rin walks towards Yukio and curls up next to him. “What will his tail look like?”

“Well, we think that it’s going to be brown like his hair and possibly have feathers in random places,” 

“That’s Cool, is he some kind of bird demon?” Rin mutters as he falls asleep.

“That’s right. Now you look exhausted so you sleep and we will continue this in the morning,” Ibis says shutting off the light.


	8. Chapter 8

It was on the first day of Yukio’s tail growing that it clicked with Rin that his brother was in a lot of pain and he couldn’t help besides cuddling up to him and act as a small heater. So far his tail was just a lump that was pushing its way out. Yukio is allowed to walk around the dorm but he wasn’t allowed to leave.

By the second day, it was pretty obvious what was happening the tail was around a foot long and was covered in a thin layer of down feathers. Which Mephisto said might turn into regular feathers or just have them in some spots. Yukio was still allowed to move around but now he had to have someone with him at all times. In the middle of the night he had thrown up, Iblis had gotten him cleaned up and set him back in his bed.

On the third day, the growth rate had decreased and as such, it was now a lot more painful. Nerves had started to grow in and the tail was able to twitch. Yukio wasn’t allowed to walk around as he would crash into walls tripping over his feet.

On day four it seemed to have started to affect him in more ways as tail related instincts had started to show up. If someone entered the room suddenly his tail would jerk up causing him to whimper. He was stuck laying on his stomach unable to move.

By day five the tail was around three feet long can move pretty well. Yukio was allowed to sit up and he was moved to Rin’s bed so his sheets could be changed as there were tiny feathers everywhere. Wile on the bed Rin decided to climb on top of his lap and sit there. Hugging his brother and pushing little bits of food into his mouth. Which made Mephisto calps to the floor in laughter.

Day six the tail was completely done growing. “Hey, Mephisto what did you tell the school?” Yukio asks his tail curling around Rin’s.

“The cram school was told that your demon blood was activating and would be back, and the school was told that you got the flu and would be back in a few weeks. So you have to teach cram school but you don’t have to go to school until you can control your tail,” Mephisto says not looking up from his game.

“Oh, I feel loved,” Rin says sarcastically. “Yukio gets time to adjust, but I just get tossed into school,”

“Oh hush,” Iblis says poking Rin’s side then puts him on his shoulders. “Yukio, for now, try not to get your tail wet as it will probably be overly sensitive with all the new nerves,”

“I’ll keep that in mind,” Yukio says, he has come to terms with his demon side after thinking it over and talking to Shiro who was there until he got called to go on a mission.

  
A few days later Yukio was able to walk without tripping over his tail so Yukio heads to the Cram school with Rin to teach his lessons.

“As Mephisto told you my demon blood awakened, now this doesn’t change anything as I wasn’t sealed so I have no problem with holy objects or other things that effect demons as I am mostly human. Moving on today’s lesson is on poisons that affect humans but can act as medicine to demons, before any questions to me they are still poison, and we haven’t tested Rin as in his current state it would be like testing an old bulletproof vest on a person” Shima’s hand drops back to the desk at this. “A good example for this poison I normally would be belladonna more commonly known as deadly nightshade,”

As the lesson drags on Rin begins to grow increasingly bored and starts to fidget in his seat, until he falls out of his chair. The entire lesson stops and everyone is staring at him. “Um, sorry?” At this, the class explodes into laughter. “What?”

“Oh man, your expression when you fell,” Shima chokes out.

“Rin, take this,” Konekomaru says handing him a feather, that had been in his bag.

Rin grumbles a thank you when Mephisto sweeps into the room and make an announcement. “Hello, exwires And Yukio. I have come to you today to inform you that my brothers have declared that they will be having a reunion here. Why, I don’t know and don’t worry nobody will get hurt,” at this he disappears taking Rin with him.

“Can anyone tell me what plant is good for burns?” Yukio asks getting the class back on track.  
.............  
“So, why did you take me,” Rin asks unamused sitting cross-legged in the air, eye level with Mephisto.

“Well you've been cursed for two months now and I’ve noticed that your mental state is very child-like, so I’ve assigned Kuro to watch you and keep track of you.”

“How is that gonna work? Last I checked the cram school won’t allow familiars to come.”

“Well yes, but Kuro had a more human form but after spending to much time with humans he lost that form,”

“What does that have to do with anything?”

“Well, in short, I gave Kuro his human form back”

“Really? That’s awesome!”

“Yes, yes it is. He is still at the dorms getting used to being human shaped. He asked me to explain demon forms to you. So I will, lower levels like coal tars have the one form, mid-levels have two a demon form and normally an animal-like form. As for us high levels we have several. We have the human form, what you looked like before getting cursed, a demon from which you are slowly getting and an animal form we are special though and are able to do most animals.”

“Ok, but what will happen I don’t think that he’s going to want to go to cram school,” Rin says with a pout.

“Oh, you aren’t going to cram school. Not until you're back to normal. You should start getting back to normal in a month.”

 

**Notes for the Chapter:**

> Sorry it’s a bit late.


	9. 9

“Yukio, I’m bored,” Rin says for what seems the hundredth time that day. Kuro had been tutoring Rin for around two weeks and Mephisto had sent him to check up on Shiro so he was gone for a few days.

Yukio takes a deep breath. “Rin, you know what, go find someone else to bother, if you haven’t noticed I’m busy,” his tail lashing the air.

“But, Kuro left for a few days to go see dad, Meffy wanted actual food for once so Ukobach isn’t here Meffy took him with him on his trip, And Meffy told you to watch me until he gets back from his trip,” Rin wined pouting crossing his arms and flops on to his bed, making a clicking noise with his tongue.

“Rin, If you don’t stop doing that, you won’t.... you know what just shut up or leave the room!”

“Fine, I’ll leave don’t expect me to be able to do anything by my self,” Rin yells transforming into a cat and runs into the dining hall where there was a climbing tree set up. Climes up to the top of the tree and starts shredding one of the papers that had been put in the tree when they got it. “He always does that he starts paying attention to me and is nice, then he’s mean and starts yelling,” Rin mutters to himself. Curling into a ball and falling asleep.

..................  
‘Why can’t Rin understand that I’m busy he’s always doing this coming in and bugging me while I’m working.’ Yukio thinks his head in his hands and tail curling tightly around his chest. When his phone rings he listens to the speaker while grabbing some things and stuffing them in his bag.

“Mr. Okrmra we have a mission for you to go on, it is a solo mission. You will be locating the source of a coal tar infestation and destroying it,” The voice from the phone says.

“For that kind of exorcism we are supposed to have back up in case of problems,” Yukio says slowly.

“Don’t question orders boy! You were selected for this. Don’t worry about it. Your brother will be fine we will send the girls in Rin’s class to watch him.”

“Alright I’ll go, how long is this expected to be?”

“It should take Three days max,” after saying this the phone hangs up.

“Rin, I have to go on a mission. I should be back in about three days, I know you heard me and I’m sorry about earlier I was really frustrated. Izumo and Shiemi should come and get you in the morning. I love you bye,” He yells running out the door.

Little does he know that Rin had not heard him and was sleeping in the tree. By the time Rin wakes up the sun is setting and Yukio is long gone. There was a plate of food on the table left by Yukio. Rin eats the food and sets off to look for Yukio to apologize. After ten minutes of not being able to find him, Rin gets scared and starts to cry he makes his way to the room which he and Yukio share and cries himself to sleep on Yukio’s bed.

  
Morning came and Yukio hadn’t come back Rin was so worried about his brother he didn’t see the shadow move across the floor. Rin was to short to reach the counter tops in the kitchen so Yukio had stocked the bottom shelf of the fridge with food Rin would eat.  
...........  
“There’s nothing here! Why would someone send a mission file for something with nothing here,” Yukio yells tail thrashing.

“Oh, so I’m nothing now am I? Brother won’t be happy if I killed a sibling but that’s just insulting.” A voice says dripping with fake sadness.

“Who Are you!” Yukio demands spinning around seeing nothing out of place in the old warehouse.

“No one ever looks up. I’m Beelzebub demon king of insects. Nice to meet you little brother,” The now named Beelzebub says dropping down from the ceiling. He looked cute in a strange way his eyes were big and dark green. His hair was a similar color and was cut short. What Yukio was fascinated with was his tail the same color as his hair, but looked like it had the same texture as a tarantulas leg.

Yukio jumps then says “So you gave me this mission,”

“That’s right. We gave you this mission.”

“We?”

“Well yes, Egyn wanted to meet the twins but he’s so shy that he had to meet you one at a time so we came up with this, we did have to give the plan to Mephisto to be checked so it was okayed by him. So we had permission.” Beelzebub explains as he starts dragging Yukio. “Come On let's go back and see Rin.”  
............

By this point, Rin was back on Yukio’s bed and was reading a manga that was left on his bed. Egyn had built up enuff courage to talk to Rin. “Hello, Rin,”

“Hi... who are you?” Rin asks backing as close to the wall as possible.

“Oh um, I’m Egyn demon king of water and your brother.” Egyn stutters out. His blue bangs falling into his eyes.

“Brother? Nice,” Rin says hugging Egyn gently bumping him with his horns, giving him a chance to sent mark him and get to know him. “Do you know where Yukio is he went somewhere and I didn’t get to apologize to him.”

“You got in a fight?” Egyn asks.

“Well kind of, I was distracting him and he yelled at me so I went into my cat tree. Then I fell asleep and couldn’t find him,” Rin explains rambling on.

“That’s no fun, they should be back in a few hours.”

“They, Who is they?”

“Two brothers Yukio and Beelzebub, just so you know there are two brothers who don’t like you very much. I can’t tell you why but you’ve met one.”

“Asroth? That guy the king of rot,”

“Yes, that guy,” Egyn laffs. “There is one other brother who would love to meet you but he got trapped he should get out soon though,”  
...........  
By the time Yukio had gotten back with Beelzebub. Rin and Egyn were best friends.

“So, you’re Rin, I’m one of you older brothers Beelzebub,” At the same time Egyn was giving a similar introduction to Yukio.  
..............  
“Goodbye, little brothers. I’ll see you again,” Beelzebub says waving as he and Egyn leave the dorm.

“We shall meet again,” Egyn says bumping noses with Rin. “Goodbye,” As he says this they disappear into the night.

“So, Rin What just happened?” Yukio asks with a dazed look on his face.

“Well, we met two new brothers,” Rin answers poking Yukio’s leg.

Yukio bends down and picks Rin up and staring into his eyes says, “Rin, that was rhetorical.”

“Oh, can we get food now,” Rin says.

“You’re always hungry,”

**Notes for the Chapter:**

> Next time Mephisto trip!


	10. Chapter 10

**Notes for the Chapter:**

> I am so sorry that I'm so late I was really busy. Please don't be too mad

Mephisto stands from his table at Vatican headquarters. “As you know the paladin awoke from his coma-

“Yes but what of the satan spawn?” A masked figure asks standing slamming his hands on the table.

“The twins know that there adopted father is awake.”

“That was not what I asked. I asked are they dead!”

“Now, now what reason would I have to kill two innocent children? They have done nothing to anyone here in fact if I remember correctly Rin saved you from Amiamon so why do you continue to chase after them?”

“They are demons the literal definition of evil,”

“Perhaps, but they were raised human, they know nothing of what being a demon truly is,”

“Yes, But what if they were exposed to Gehenna what would happen?”

“Well I suppose the older one would be forced to a full demon form and the younger one would probably be fine with little to no change,”

“That is exactly why they must be executed!” A new voice calls out. This one with a hood covering his face.

“The mental state of the boys will not be changed, they will only change appearance-wise,” Shiro says entering the meeting hall. “Didn’t Rin save this place from Amiamon so why doesn’t that clear his name?”

“Paladin, why would you of all people defend demons?” The hooded figure asks. “Where you not put in the hospital because of one?”

“Why would I defend a demon well, I recently delivered that some demons who attack people have either been a attacked first, disrespected, hurt, or they are going through the demon equivalent to puberty. And second, those two are my kids!”

“Meetings over we will meet again in a few months to discuss this further. Mephisto I want you to test what happens if the twins are exposed to Gehenna.” The masked figure instructs.

“I’ll get right on that! eins zwei drei,” He says and with a poof, he’s back at True Cross.  
.....................  
“Yukio, Mephisto’s comes Home today right?” Rin asks from the floor in the bedroom.

“Yes, he is. Hey, are you ok?” Yukio says turning around in his chare. To look at Rin who looks like he has a headache.

“Everything hurts, not bad or anything but, it hurts!” Rin says bouncing on his feet as he hadn’t been able to release his pent up energy for a while.

“Really, it sounds like the curse is wearing off.”

“Oh! really?” Rin says excitedly before deflating.

“What’s wrong?”

“I’m not sure. I should be happy and I am but I-I Don’t know, stuff feels right, and I don’t want to lose it,”

“Hello, baby brothers!” Mephisto yells as he enters the room.

“Last I checked there is only one baby in the room.”

“Well appearance wise I’ve seen you playing with Rin’s toys, don’t you dare deny it.” At this Yukio turns a bright red.

“Meffy!” Rin yells throwing himself at Mephisto.

Mephisto catches Rin and asks. “So what happened while I was gone?”

“Oh, it was fun! We met Bezlebub and Egyn.” Rin says clinging to Mephisto’s arm.

“The curse is starting to fade,” Yukio says having composed himself.

“That’s good,” Mephisto says suddenly serious. “You know how I had a Vatican meeting, well I’m supposed to give both of you essence of Gehenna.”

“Is that bad?” Rin asks.

“For you, maybe, but Yukio shouldn’t have a problem other than temporally sharpened fangs, ears, and nails much like how you looked with the Koma sword drawn.”

“When are you going to do this?,” Yukio asks tail flicking a bit nervously.

“Oh, three weeks or so. After Rin’s normal and functional.” At this point, rin was completely exhausted he was nearly passed out in Mephisto’s arms.

“Ok, so what now?”

“Well its almost night so Rin should sleep for the rest of the day. The first week of him growing up will be relatively simple he will sleep for much of it. It’s the second week when he’s around eight that it starts to be difficult,”  
..............  
“Hey, Yukio my lisp is gone!” Rin yells as he jumps on his brother's bed and on to his brother.

“It’s a Saturday go back to sleep,” Yukio grones as he’s jolted awake.

“But I can speak I don’t- OW, Don’t look so panicked I bit my tongue,”

“Rin, get off of me,” Yukio grunts out.

“Ok,”

After Yukio was awake enough to function he says “We are going to Mephisto’s to make sure that you aren’t getting hurt by this,”  
...........

“So Rin, You went from a one-year-old to a three-year-old,” Ibis says, “The first week is easy but after that, it will get a little hectic,”

“What do you mean?”

“Well in human terms when you start your wonderful journey to adulthood,”

“Nope!” Rin yells and tries to run out the door only to get grabbed by the back of his shirt and dragged back by Ibis.

Mephisto comes out of nowhere. “If you hate it that much little brother I’ll put you to sleep for that part,” at this he raises his hands and pauses. “you know what I’ll just do that for the entire process,” after he says this he claps his hands, gets pink glitter from somewhere, and throws it over Rin.

“What, what did you do?” Rin asks slurring heavily.

“I made it so you would sleep until you at your proper age,” Mephisto explains as Rin drifts off.

“Why the heck would you do that!” Yukio yells his tail lashing the air.

“Shush, you,” Mephisto exclaims bopping his nose, “do you want Rin to wake up? ‘Not that he could wake up before the two weeks are up’”

“What was that?”

“Nothing” at this Mephisto teleports out of the room leaving Yukio with Rin.  
............................  
“So, any reasons for putting him to sleep beside less work for you?” Ibis asks Mephisto who was sitting at his desk building a card house.

Who shrugs “less work for everyone, it’s easier in the long run,”

“Meaning what exactly?”

“Well, for starters he won’t be in any ones way,”

“You did this just so he wouldn’t annoy you didn’t you,”

“Fine you caught me, I didn’t want to be so overwhelmed with him that I wouldn’t have time for anything else,”

“You are an interesting one brother, unfortunately, I have to leave now I should see you in about two weeks. Don’t mess up.”

“Who me never,” Mephisto says with false innocence. “Don’t die while you’re gone,”

...........................  
Two weeks later

“Good news, Rin will wake up today! Bad news Rin will have to rebalance and practice his powers. You'll get him back in a day or so. -Mephisto,” Yukio reads aloud. “I should have expected this.”  
.......  
“So Rin how are you feeling?” Mephisto asks as Rin sits up rubbing his head.

“Like I got hit by a bus. Holy crap my voice!” Rin exclaims then bursts out laffing “I forgot how my voice sounded”

“Yes, yes that happens. Now, will you try to shapeshift?”

Rin nods and shifts into a cat that was a little bigger than Kuro. “What now?”

“Turn back and flote”

Rin complies turning back and hovers a few feet off the ground.

“Good, now try to walk,”

“Ow,” Rin says after standing and then falling on his butt.

Mephisto bursts out laffing nearly falling off his desk. “Oh, that was amazingly graceful. I applaud you, your highness,”

“Hu? Why did you call me that?”

“Well, you are a prince,”

“Well, you’ve never called me that before,”

“Hush you. That’s not important right now, try to walk again,”

Rin stands up slowly than before and shakily takes a few steps and starts walking around until he gains his balance,”

“Ok, you may go back to your dorm,” Mephisto says flicking his fingers towards the door,  
......................

“Yukio I’m back!” Rin calls out to his brother.

“Rin, welcome home, there should be a package by the door,” Yukio says happily sitting down in the dining room.

“I see it you want me to get it for ya,” Rin says pick it up and looking inside it. “It’s a weird liquid thing?”

“Do you remember what Mephisto was as talking about before you changed back? The essence of Gehenna he said that he’ll come and test it out in a few days so you have time to Adjust,”

**Author's Note:**

> *(Think king from Seven Deadly Sins)  
> **(Kuro's fang is what lets him turn into a human)  
> If you do not know what Kuro's human form looks like look it up, it’s really cool. If you have suggestions for some instincts please comment.


End file.
